Trunk piston engines are generally medium speed (300-1000 rpm), 4-stroke engines, in which a single lubricating oil is employed for lubrication of all areas of the engine, as opposed to the crosshead engines in which the crosshead allows for use of separate lubricants in the cylinder and in the crankcase. A trunk piston engine oil (TPEO) therefore has unique requirements for fuel compatibility, oxidative stability, viscosity increase control, and detergency.
Traditionally fuel oils used for the operation of trunk piston engines have ranged from heavy marine residual fuel to low sulfur distillate fuel. Recently, driven by health and environmental concerns, there has been increasing probability of future regulations mandating the use of low sulfur fuel for the operation of trunk piston engines. The use of low sulfur residual fuel requires that it is feasible for refineries to lower the sulfur level in residual fuel at a reasonable cost and effort. It is unknown whether there will be sufficient low-sulfur residual fuel oil available in the future, or whether low sulfur distillate fuel and gas oils will be used to a wider extent. It is therefore desirable to provide a trunk piston engine oil composition designed for use with low sulfur distillate fuel where the lubricating oil has a low base number but is capable of providing oxidative stability, viscosity increase control, and improved detergency performance.
Additives, especially metal-containing alkaline detergent additives, have been used for many years in TPEOs to neutralize acid combustion gases, maintain engine cleanliness, ensure compatibility of the lubricant with residual fuel oil, and control viscosity increase. However, it remains unclear if TPEOs formulated with additive technology developed for use with residual fuel oils will in fact be optimum for the low sulfur distillate marine fuels of the future due to differences in the characteristics of the fuels and differences in the environment of trunk piston engines due to the varying sources of fuels.
Key performance parameters for operating trunk piston engines with either heavy marine residual fuels or low sulfur distillate fuels include: deposit control of the piston cooling gallery (also known as the undercrown), deposit control of the piston ring pack (this includes the rings, lands and grooves), viscosity increase control, and sludge control. For marine residual fuels operation, these performance parameters are almost exclusively driven by asphaltenes contamination from the marine residual fuels.
For distillate fuel operation, however, where the fuel contains no significant asphaltenes these performance parameters are driven by combustion by-products from the distillate fuel. Therefore, the requirements for engines operated using low sulfur distillate fuels versus marine residual fuels are very different. As a result, this does not allow for performance read-across of a formulation from distillate fuel operation to marine residual fuels operation or vice versa. A formulation that is specifically designed to offer optimum performance for marine residual fuels operation would not be expected to automatically provide acceptable performance for distillate fuel operation. For example, down-treating traditional 70 TBN Marine Cylinder Lubricant formulations optimized for high sulfur marine residual fuels operation to 40 TBN Marine Cylinder Lubricant formulation have been shown to have unacceptable performance for low sulfur Marine Cylinder Lubricant operation. Further, down-treating traditional 70 TBN Marine Cylinder Lubricant formulations optimized for high sulfur marine residual fuels operation to even lower TBN Marine Cylinder Lubricant formulations have also been found to have unacceptable performance for distillate fuel operation.